This chapter reviews the changing epidemiology of plague over the past several decades and discusses factors of plague emergence or re-emergence. Three biotypes of Yersinia pestis, classified according to their ability to ferment glycerol and reduce nitrate, are correlated with the three major plague pandemics of history. Effective strategies to prevent and control the spread of plague were made possible by an understanding of its ecology and epidemiology. Epidemiologic investigations revealed that, with rare exception, human-to-human plague transmission occurs only as a result of close contact of persons with a case of pneumonic plague and that contagious spread could be interrupted by isolating plague patients. Many of the natural foci of plague are being increasingly invaded by humans and are undergoing dramatic man-made environmental changes. Under some circumstances, a changing rural environment such as suburbanization in the southwestern United States may increase human risk. An important factor today in the epidemiology of plague is its entrenchment in natural rural cycles. The ability to detect, monitor, and respond to plague is based on a solid public health infrastructure. In addition to routine laboratory studies, there is an important role for advanced genetic techniques to define the molecular epidemiology of plague and to detect and characterize newly evolving traits, such as virulence factors and factors associated with antimicrobial resistance.

Plague cases reported to the World Health Organization, by geographic region, 1948 10 1994, showing marked decrease in cases in Asia and a recent increase in cases in Africa (note difference in scale for cases).

10.1128/9781555816957/fig9-4_thmb.gif

10.1128/9781555816957/fig9-4.gif

Figure 4

Plague cases reported to the World Health Organization, by geographic region, 1948 10 1994, showing marked decrease in cases in Asia and a recent increase in cases in Africa (note difference in scale for cases).

Human plague cases, United Slates, 1900 to 1994, by 5-year intervals, showing peak activity during urban plague outbreaks in the early 20th century, a period of quiescence, and recent resurgence due to increased rural plague in south-western states.

10.1128/9781555816957/fig9-6_thmb.gif

10.1128/9781555816957/fig9-6.gif

Figure 6

Human plague cases, United Slates, 1900 to 1994, by 5-year intervals, showing peak activity during urban plague outbreaks in the early 20th century, a period of quiescence, and recent resurgence due to increased rural plague in south-western states.

Number of human plague cases reported by state and decade in the United States from 1944 to 1993 (total, 362 cases), showing increasing numbers of cases, increasing number of states reporting cases, and an eastward shift in state of occurrence.

10.1128/9781555816957/fig9-7_thmb.gif

10.1128/9781555816957/fig9-7.gif

Figure 7

Number of human plague cases reported by state and decade in the United States from 1944 to 1993 (total, 362 cases), showing increasing numbers of cases, increasing number of states reporting cases, and an eastward shift in state of occurrence.

35.Schwann, T. G.,, D.Thompson,, and B. C.Nelson. 1985. Fleas on roof rats in six areas of Los Angeles County, California: their potential role in the transmission of plague and murine typhus in humans. Am. J. Trop. Med. Hyg.34:372–379.